EFFECTS OF CLIMATE EVENTS ON ELEMENTAL FLUXES FROM FORESTED CATCHMENTS IN ONTARIO

One of the principal influences on the flux of many elements from forested catchments in Ontario is the atmospheric deposition rate of strong acids. While sulphate deposition has decreased ca. 45% in the past 2 decades, nitrate deposition has remained unchanged and now equals sulphate deposition. Sulphate concentrations in headwater lakes and their inflows have decreased, but much less than expected based on the anticipated direct response of the catchments. In Harp and Plastic Lakes, the decrease has been 28 and 21%, respectively. while for others, the decrease has been even lower. As a further consequence, alkalinity and pH recovery has been slow. We have identified redox processes occurring in wetlands as the reason for delayed recovery, and climate events, particularly the El Nino phenomenon, as controlling these redox processes. A new version of the biogeochemical model MAGIC that incorporates wetlands and redox processes driven by climate events has been generated, and model predictions show good agreement with field measurements.